With the gradual enhancement of the requirements for capacity and flexibility of the optical communications system, coherent optical communications technology becomes more and more important. In comparison with incoherent technologies (such as on-off key, OOK) or self-coherent technologies (such as differential quadrature phase shift keying DQPSK), the coherent technology has the following advantages: a gain of 3 dB optical signal-to-noise ratio (OSNR), the possibility to employ more efficient modulation technologies (such as quadrature modulation, QAM) to enhance transmission capacity, the convenient use of electric equalization technology to respond to changes in channels, and to reduce production costs, etc.
The structure of an optical coherent receiver is also complicated relative to the traditional incoherent or self-coherent receiver. For example, besides the traditional optoelectric detection module, the optical coherent receiver must also include a phase recovering module and an adaptive electric equalization module, etc. The complicated structure and the adaptive characteristics of the receiver demand that the physical layer working performance of the receiver be monitored so as to quickly position and remove failures. However, performance monitoring of an optical receiver is traditionally carried out by statistics about the error rate (namely, bit error rate) of the finally recovered digital bits, and such statistics is performed after the transmission frame structure has been recovered. Accordingly, the bit error rate obtained by such statistics is further subjected to influences of non-physical layer factors (such as the multiplexing mode of the receiver and the frame structure of the transmission data, etc.), so that the bit error rate cannot directly and exactly reflect the physical layer working performance of the receiver. Moreover, statistics about the bit error rate usually takes a considerable lot of time, fifteen minutes or an hour for instance, and such lengthy period of time is obviously to the detriment of failure positioning and removal. There is hence an urgent need for an apparatus and a method to quickly, precisely and directly monitor the physical layer working performance of the optical coherent receiver.